Do you like the well-known Echo joke “Who’s King Weasel?” Try it on a time-reflecting pit, the answer won’t be “donkey!” , but “Read!” , as if you were playing a disc in reverse. By the way, the physicists did not build a well during their research, but rather a “mirror” that reflects electromagnetic waves – light – in an inverted time sequence. Or, more precisely: a six-meter board covered with metal threads and electronic switches.

“An ordinary mirror rotates left and right, and a phase coupling mirror, as we call it in practice, reverses time,” says physicist Carlo Bennaker of Leiden University, who was not involved in the research.

Although such technologies have been around for some time, until now they have been limited to very narrow wavelength ranges, he says. As if your mirror only works on one very specific shade of color. In their new research, the physicists have achieved the same for light in a much wider range of wavelengths, a stepping stone to actually using these mirrors in practice.

“something magical”

“Make no mistake: As exciting as a time reversal may sound, this is not a time machine or anything,” says Benaker. At the same time, he describes what is possible with these mirrors as “something magical.”

The most famous application is used to remove disturbances from light. In experiments, you can then shine a beam of light through a cloudy object, so that the light scatters. “A glass of milk, for example,” he says. By letting the light fall on a time-reflecting mirror, you can then eliminate the scattering of that light.

People often say: If only I could return the movie, undo this bug. Unfortunately, this isn’t possible with humans, but something similar can be done with light. In practice, this effect is useful, for example, for astronomical observations from Earth, when you need to free light from the deep universe from disturbances caused by the Earth’s atmosphere.

Red turns green

Although the physicists performed their experiment on light invisible to the naked eye, the result of the time mirror on ordinary light would be strange. Because the frequency of this light changes according to the technology used, you will see the colors reflected differently in these physicists’ time-reversing mirror: red becomes green, orange becomes blue and yellow appears magenta.

The fact that it has now succeeded for the first time in providing a time-reflection of a wide wavelength range is because the researchers made the mirror out of metamaterials. Such materials have counterintuitive optical properties that are also used in the development of “invisibility cloaks,” where they bend light around an object so that you can’t see something.

According to the researchers, the ability to manipulate electromagnetic waves with metamaterials not only in their spatial motion but also in time could lead to more efficient radio communication and improved radar technology in the long term, among other things.